margin:auto; with inline-block

margin:auto; with inline-block

i have a "container" div to which i gave margin:auto; which worked fine as
long as i gave it a specific width, but now i changed it to inline-block
and margin:auto; isnt working
previous css
#container {
border: 1px solid black;
height: 500px;
width: 650px;
}
.MtopBig {
margin-top: 75px;
}
.center {
margin-left: auto;
margin-right: auto;
text-align: center;
}
new css
#container {
border: 1px solid black;
display: inline-block;
padding: 50px;
}
.MtopBig {
margin: 75px auto;
position: relative;
}
.center {
text-align: center;
}
DEMO fiddle.